Thanks for all the input everyone.
What' I've decided to do was to get the
VeloLogic rim Dynamo and the
Sinewave Revolution USB charger.
This should allow me to charge when I want/need to and shut the generator off entirely when I don't. I can swap it to another bike easily (EDIT: or the other wheelset on the same bike) if I'd like to use a different bike on a trip. If I want to add a light into the system later I can. Its also slightly less expensive then a generator hub and a lamp that also charges USB devices.
EDIT: This option is also lighter than a dyno hub, even if you figure in a ~3000mAh back-up battery for charging on days off the bike.